EcosystemsThe organization of the biosphere, from the broadest level of organization to the most specific level of organization, is shown below:

biosphere → biome → ecosystem → community → population → organism

BiosphereThe biosphere includes any part of the Earth where organisms live. It extends from the crust of the Earth to the atmosphere. All of the ecosystems on the Earth are included within the biosphere.

All of the ecosystems on Earth are part of the biosphere.

The biosphere is the broadest level of ecological study. It includes interactions between different ecosystems that can only be studied by viewing the entire Earth as one large system. The lithosphere, hydrosphere, and atmosphere are all part of the biosphere.

BiomeA biome is a geographic region that has a distinct climate. A biome is made up of separate (but similar) ecosystems, so it contains characteristic types of plants and animals adapted to the region and its climate.

Deserts, grasslands, and rainforests are all examples of biomes.

EcosystemAn ecosystem is made up of the biotic, or living, community and its abiotic, or nonliving, environment. Abiotic factors include rocks, air, dirt, sunlight and water.An example of an ecosystem would include all of the living and nonliving factors that are inside a pond. The water in the pond, the algae and plants that grow in the water, the animals and bacteria that live in the water, the dirt and rocks on the bottom of the pond, and the sunlight that hits the water would all be considered a part of this ecosystem.

All of the living and nonliving factors of this coral reef are part of the coral reef ecosystem.

Ecosystems can vary greatly in size and conditions. The abiotic factors that make up an ecosystem determine what kinds of plants and animals that can live there. For example, a desert ecosystem that is very hot and does not receive much water can only support certain kinds of organisms, such as cacti and lizards.

CommunityA community includes all of the populations that live and interact in the same area. An example of a community is all of the plants and animals inside of a forest. All of these organisms interact and depend on one another for survival.A community makes up an ecosystem's living, or biotic, portion.

All of the plants and animals in this forest belong to the same community.

At the community level, interactions between organisms can be observed. For example, predator/prey and consumer/producer interactions occur at this level. Competition and cooperation between different species are also part of community ecology.

PopulationA population is a group of individuals of the same species living in the same area at the same time. An example of a population of organisms is a grove of orange trees.

All of the orange trees in this grove belong to the same population.

Populations can be defined at different levels of size. For example, a local population could occupy a very small habitat, such as a puddle. A population could also include every member of a species of monkey that occupies a large island. There is generally a boundary between populations of the same species, such as an ocean or an area of land that the animals do not freely cross.

Changes in EcosystemsThe survival of organisms depends greatly on physical factors in their environment. Any changes to either the biotic or abiotic factors can impact an ecosystem. Some changes that might appear to be minor could have a large effect. For example, minor changes in the pH of a body of water can cause massive fish kills.But not all changes are negative, some could be positive. By simply moving through the soil, worms are able to help break up the soil and add air. This improves the quality of soil, which plants benefit from.Changes in an environment also have ongoing effects. For example, if some or all of the plants in an ecosystem were to die, the animals that feed on the plants would have less food, so fewer of the animals would survive.

As the population of these primary consumers becomes smaller, secondary consumers would begin to compete with each other more for food, and this population would also begin to shrink.

Biotic & Abiotic FactorsFactors which can affect change in a population or species are usually divided into two types: abiotic and biotic. Abiotic refers to the nonliving environmental factors, while biotic refers to the influence or effect created by a living organism. 

Both abiotic and biotic factors can limit or enhance a population’s success in a particular environment.

Biotic FactorsThe word biotic comes from the Greek word for "life". Biotic factors include not only living organisms but also factors from formerly living organisms.

Biotic factors include both living things and things that were once living. This means that the cotton plant and a cotton shirt are both

considered biotic.

For example, both a cotton plant and a cotton t-shirt would be considered biotic factors because they are derived from the same living organism. Biotic factors typically influence the viability of a community and include interactions between members of the same species and members of different species.

Abiotic FactorsAbiotic factors include any nonliving geological, geographical and climatological factors. Specific abiotic factors are water, air, soil, light, temperature, and natural disasters. These things are a part of each species’ ecological niche because they influence how populations affect, and are affected by, resources and enemies.

Abiotic factors shown here include air, water, light, soil, and even the forest fire.

BiomesThe term biome refers to a geographic region that has a distinct climate. A biome contains characteristic types of plants and animals adapted to the region and its climate.

Characteristics of BiomesThe climate of a location determines which types of organisms are able to live there. Climates that are very cold are home to plants and animals that have adapted to the extreme temperatures. The same is true for climates that are extremely hot and dry.The major biomes on Earth include: tropical rainforest, temperate rainforest, desert, grassland, deciduous forest, coniferous forest, tundra, estuary, savanna, and taiga.

Tropical RainforestThe climate of a tropical rainforest is hot and wet. Heavy rainfall (around 150 cm per year) and year-round warm temperatures make it very humid. This climate is found near the equator. A tropical rainforest is very dense with lots of large trees that block out sunlight. Very little sunlight reaches the rainforest floor.

Tropical rainforests are very hot and wet.

Temperate RainforestThe climate of a temperate rainforest is mild and wet. Temperatures are moderate and change with the seasons. Rainfall amounts are very high. Like tropical rainforests, temperate rainforests have a thick canopy of trees that block most sunlight from hitting the forest floor. However, while tropical rainforest canopies are generally broadleaf trees, temperate rainforest canopies may be broadleaf or coniferous trees.

Temperate rainforests commonly have coniferous canopies, with smaller broadleaf plants in the understory.

Image courtesy of NPS.

Temperate rainforests are farther from the equator than tropical rainforests and can be found in the U.S. Pacific Northwest, Asia, South America, Europe, and Australia.

DesertThe climate of tropical deserts is generally hot and dry. However, temperate deserts, such as the Gobi in Asia, are much cooler. The amount of precipitation in all deserts is less than the amount of water that could potentially evaporate. Deserts get less than 25 centimeters of rain every year.Desert plants and animals are adapted to store water and withstand year-round hot temperatures.

Deserts are very dry and generally hot.

Succulents, snakes, scorpions, and mice are examples of organisms that live in desert ecosystems.

Temperate GrasslandsTemperate grasslands are located in the dry temperate interiors of continents.In North America, they are called prairies, in Asia they are referred to assteppes, and in South America they are pampas.Temperate grasslands receive enough rainfall to support grasses, but not enough to support the growth of large trees. The temperatures are generally warm in the summer and cold in the winter. Drought and wildfires are also common in this biome.

The temperate grassland is characterized by thick, mixed grasses.

Prairies are often converted into farmland due to the rich soils found there.Some of the animals that can be found in this ecosystem include grasshoppers, prairie dogs, and bison.

Deciduous ForestThe climate of a deciduous forest is temperate with four distinct seasons (spring, summer, fall, and winter). Deciduous forests have warm summers and cold winters. They have moderate precipitation throughout the year.

Deciduous forests have four distinct seasons.

During winter months, however, the precipitation is usually frozen and unavailable to the organisms that live there. Trees in a deciduous forest usually lose their leaves during the winter and have thick bark to conserve water and protect them from the cold.

TaigaThe taiga, known also as a boreal forest, is the largest continental biome. It experiences long, cold winters; short, warm summers; and low precipitation. It is characterized by coniferous forests. Taiga covers most of Canada and Siberia and is not found in the Southern Hemisphere.

The taiga is characterized by coniferous forests; long, cold winters; short, warm summers; and low precipitation.

Arctic foxes, wolves, and snowy owls are a few examples of the animals that live in the taiga.

TundraThe tundra has very low temperatures and very little precipitation. Winters in the tundra are long and extremely cold; summers are short, mild, and cool.The animals living in tundra ecosystems have adaptations that allow them to stay warm in the very low temperatures. For instance, Arctic foxes, grizzly bears, and ermines (a kind of weasel) all have thick fur that protects them from the cold.

The tundra is very cold and dry.

Tundra is characterized by its frozen subsoil, which makes only a small layer of soil available to plant life. This limits the plants that can grow to small low growing plants such as mosses and grasses and makes it impossible for trees to grow.

Estuary

An estuary is an area in which fresh water and salty ocean waters mix together. These areas may include bays, mouths of rivers, salt marshes, and lagoons. These brackish (salt mixed with fresh) water ecosystems shelter and feed marine life, birds, and other wildlife with nutrients from the ocean.

Estuaries provide a place where young animals can grow up.

The plants in estuaries are marsh grasses and other plants that are adapted to water levels that change with the tides. Muskrats, herons, egrets, shrimp, and crabs are animals often found in estuaries.

SavannaA savanna is a dry tropical grassland where trees are present but more widely scattered than in forest ecosystems. The savanna climate has a temperature range of 68° to 86°F.Savannas receive around 125 centimeters of rain every year, with most of the rain falling during the summer. Because of the vast differences in precipitation, the summer is referred to as the "wet season," and the winter is the "dry season."

The trees in a savanna are very sparse and spread out.

Zebras, giraffes, lions, and acacia trees are found in the savannas of Africa.

Ecosystems - Trophic LevelsThe organization of communities is based on the use of energy from the Sun within a given ecosystem. Organisms within a community are dependent on the survival of the other organisms because energy is passed from one organism to another.

The Sun's energy cycles through ecosystems from producers through consumers and back into the nutrient pool through decomposers.

A trophic level describes the feeding level of an organism. Producer, decomposer, primary consumer, secondary consumer, and tertiary consumer are all trophic levels that can be used to describe an organism's place in an ecosystem.

Producers

Producers are organisms that are able to synthesize food molecules from inorganic compounds. Green plants, such as sunflowers, are producers because they can make their own food using energy from the Sun during a process calledphotosynthesis. Other producers include algae and some kinds of bacteria and protists.

All other organisms in an ecosystem depend on producers for energy.

ConsumersConsumers are organisms that get energy by feeding on producers (e.g., plants) or other consumers.There are three main kinds of consumers: primary consumers, secondary consumers, and tertiary consumers. The classification given to a consumer depends on where it is located in a food chain. Primary consumers are animals that eat producers. Primary consumers are also called herbivores because they

eat only plants. A deer that eats only leaves and grass is a primary consumer and an herbivore. Secondary consumers are organisms that eat primary consumers. Secondary consumers can be carnivores if

they eat only animals. Or, they can be omnivoresif they eat both animals and plants. A wolf that kills and eats a deer is a secondary consumer and a carnivore.

Consumers eat other organisms. Deer are primary consumers. Wolves are secondary consumers. Tertiary consumers eat secondary consumers, and they may be carnivores or omnivores. A bear that eats a fish

that has fed on other fish is a tertiary consumer.Since the bear also eats berries and other plants, it is an omnivore.

Decomposers

Decomposers are organisms that consume dead organisms. As they break down dead organic matter, decomposers release nutrients back into the soil, water, and atmosphere. 

The role that decomposers play in an ecosystem is crucial. Decomposers are important for the carbon, nitrogen, phosphorus, and oxygen cycles. The nutrients that decomposers release into the soil are also used by producers to make complex organic molecules. Fungi, such as mushrooms, are examples of decomposers. Some kinds of bacteria are also decomposers.

The Flow of Energy Through an EcosystemThe ultimate source of energy for all ecosystems is the Sun.Only producers can convert solar energy into food energy.Therefore, all other organisms depend on producers for their energy needs.

Food ChainsA food chain describes the feeding relationships and energy flow between species within an ecosystem.Producers receive energy from the Sun and make food. Producers are the beginning of a food chain because all of the other organisms in the food chain depend on the food energy that is made by producers. The next organisms in the food chain are primary consumers, which eat producers. Next comesecondary consumers, then tertiary consumers, and so forth until the top carnivore is reached. All organisms in the food chain are decomposed by decomposers.

This food chain shows the flow of energy from a producer, algae, to the consumers in the ecosystem. Minnows are primary consumers,

salmon are secondary consumers, and bears are tertiary consumers.

The arrows in a food chain or a food web represent the direction of energy flow. The arrow points from the organism that is being consumed to the organism that is receiving energy. For example, in the food chain

above, the arrow points from the algae to the minnow. This means that the minnow is consuming the algae and receiving energy.

Food WebsA food web is a group of interconnected food chains. Organisms within a food web can belong to more than one trophic level, or feeding level. For example, in the food web below, krill are both primary and secondary consumers. Krill are primary consumers because they eat phytoplankton, which are producers.Krill are also secondary consumers because they eat carnivorous zooplankton, which are primary consumers.

An Antarctic food web is shown in the picture above. Organisms in food webs can belong to more than one feeding level.

Energy PyramidAn energy pyramid is a diagram that shows the relative amounts of energy located within each trophic level. The trophic levels are stacked one on top of another, with the producers on the bottom. Each level in an energy pyramid has less energy available to it than the level below.Most of the stored energy in an ecosystem is in plants and other producers.This is because most of the energy in an energy pyramid is used or lost as heat energy as it moves up the pyramid. In fact, only about 10% of the energy produced at each level is available to the one above it. This is the reason that consumers in an ecosystem cannot outnumber producers and predators cannot outnumber prey.

The size of each level of the energy pyramid is determined by the amount of energy stored in the organisms at that trophic level.

An average of only 10% of the energy from the previous level moves to the next level. The rest is used up or lost as heat energy.

Organism InteractionsOrganisms within an ecosystem interact with one another in many different ways. These interactions play an important role in the survival of the organisms and the function of the ecosystem. 

Organisms can affect one another directly, through a shared resource, or through common enemies. Some interactions are harmful to the organisms involved, whereas others provide benefits for one or both of the organisms.

Energy RelationshipsFood chains and food webs describe the feeding relationships between species and represent the flow of energy through an ecosystem. The Sun provides energy to producers, such as plants. Producers convert this energy into a form that can be eaten by consumers. Animals that eat producers are then consumed by animals at higher trophic levels. Eventually all organisms are broken back down into nutrients by decomposers.

The food chain above shows the flow of energy from a producer, a plant, to the consumers in the ecosystem. Decomposers may feed on

organisms at any stage of the food chain.Decomposers recycle nutrients back into the ecosystem.

SymbiosisSymbiosis is an interaction between individuals of different biological species.At least one of the organisms receives a benefit from the interaction. The other organism can either receive a benefit, be harmed, or not be affected in any way.A symbiotic interaction involves a close relationship between the two organisms involved. There are three main kinds of symbiotic relationships:commensalism, mutualism, and parasitism.

Mutualism

Bees receive nourishment from flowering plants, and plants are pollinated by the bees. Therefore, the relationship ismutualism.

Both organisms benefit in a mutualistic relationship.Flowers and their pollinators are one of the most common examples of mutualism because many kinds of plants depend on insects, such as moths, bees, wasps, and beetles, to perform pollination in order to reproduce. Plants that rely on pollinators attract the pollinator by the shape, color, or smell of their flowers.As the pollinator feeds on the nectar or pollen from the flower, some of the pollen sticks to its legs and body. When the pollinator visits a second plant of the same species, the pollen from the first plant is transferred to the reproductive organs of the second plant, and pollination occurs. Both organisms receive a benefit from this interaction. The pollinator receives access to a food source and the plant is able to reproduce because of their relationship.

Commensalism

Cattle egrets follow behind large grazing herbivores and eat the insects that are stirred up. This is a form ofcommensalism.

One organism benefits and the other is neither helped nor harmed in a commensalistic relationship.

Often, the benefit that the organism receives is the ability to find food more easily or protection from other organisms.

For example, large grazing herbivores, such as cattle and horses, often stir up insects as they graze on grass in fields and pastures. Birds known as cattle egrets often follow behind the grazing herbivores and eat the insects that have been displaced. Since the cattle egret benefits by being able to find food easily and the grazing herbivores are not affected by the presence of the egrets, their interaction is an example of commensalism.

Parasitism

This mosquito is receives nourishment, while the human is harmed. This is an example of parasitism.

Image is courtesy of the CDC.

One organism benefits and the other is harmed in a parasitic relationship.The organism that receives a benefit is known as aparasite. The organism that is harmed by the relationship is known as the host. The host species is usually impaired slowly over a long period of time.Parasites are usually smaller than the host species, but not always, and can live either inside the body of their host or externally. Common external parasites include fleas and mosquitoes which feed on the blood of their hosts. Internal parasites, such as tapeworms, live inside the body of their host and absorb nutrients from the host's body. In both cases, the parasite receives nutrients at the expense of the host and the host can no longer use these nutrients for its own life processes.

PredationPredation is a type of interaction in which a predator hunts, kills, and eats itsprey. Predators use the prey as a source of food. Predation is different from parasitism because the prey is killed immediately for consumption. During parasitism, the host is kept alive for a long period of time so that the parasite can

continue to receive nutrients from the host. An example of predation is a wolf pack hunting, killing, and consuming a deer.

Lions kill and eat zebra. Lions are predators. Zebras are prey.

CompetitionSince there are limited amounts of resources in an ecosystem, if one organism gets a particular resource, another does not. This leads to competition as two organisms try to access the same resources. Food, water, sunlight, and space are examples of resources that organisms compete for.Plants and animals of the same species may compete for resources such as food, water, shelter, and space.Populations of different species will also compete with one other if their needs are the same as the needs of another population in that ecosystem.For example, trees in a forest compete for sunlight. As one tree grows taller, the shorter trees are shaded by it, and they receive less sunlight. The shorter trees may die as a result.

The tall tree and the shorter trees in this picture are competing for sunlight.

CooperationCooperation is a type of interaction in which organisms work together. Many species exhibit cooperative behavior, including horses, dolphins, lions, and ants. Animals that exhibit cooperative behavior often live, travel, and/or hunt in herds or groups. Living in these groups can provide protection for the animals and a higher success rate during hunts.

Groups of organisms that live together cooperatively are usually part of ahierarchy of leadership. Some members of the group have a higher status than other members of the group. Dominant members determine what the group will do and subordinate members follow their lead. This helps to eliminate aggression between group members and allows the group to work together for the benefit of all.An example of an animal that lives in a cooperative group is the wolf. Wolves live in packs that usually include six or seven members. There are two leaders within the group, the alpha male and the alpha female, and these two pack members determine when the pack hunts, moves location, or stays in an area.The leaders of the pack are usually the pack members with the most experience in hunting and defending territory. The other pack members have roles within this pack to help the pack work effectively.

Carrying CapacityAn ecosystem can only support a limited number of living things.This number is its carrying capacity.

In any particular environment, the growth and survival of living things depends on the physical conditions and the materials available. For living things to grow and survive, they must have the correct: temperature range

amount of minerals

amount of soil

amount of air

amount of space

amount of food or sunlight

Although populations of living things could grow to be unlimited in size, they may not have enough resources to do so. The carrying capacity, or the actual number of living things that an ecosystem can support, is limited by the available energy, water, air, space, food, and minerals. It is also limited by the ability of the ecosystems to recycle dead organisms through the activities of decomposers, such as bacteria and fungi.

Whenever the population of one kind of organism is greater than the carrying capacity, the population decreases. When the population is less than the carrying capacity, the population increases. This graph gives an "ideal" example of how the population of one kind of animal is related to the carrying capacity. In this case, the carrying capacity is 500 individuals.

Population Growth FactorsThere are a number of biotic and abiotic factors that affect the growth of a population.

PopulationsA population is a group of same-species organisms living in a particular location. The number of individuals within a given population is the population size.Population density refers to the number of organisms in the population divided by a regular unit of area or volume. For instance, if there were ten fruit flies in a 10-milliliter test tube, the population would be 1 fly/mL (one fruit fly per milliliter).

Population Growth Patterns

Exponential Growth

Exponential Growth

Exponential growth is often graphed as a J-shaped population growth curve.For a population to grow, it must have a non-hostile environment and sufficient food and space. When these needs are met, populations grow in an exponential pattern. That is, the population grows at a faster and faster rate.

Carrying Capacity

Stabilized Growth

When population growth is exponential at first, and then it levels off at the ecosystem's carrying capacity, the graph is represented as an S-shaped population growth curve.Populations growing exponentially tend to expand to the point that the available resources (food and space) cannot support additional organisms.The largest population that can be supported by the resources in an ecosystem is called the carrying capacity.

Population Growth FactorsAs discussed earlier, populations with a proper environment and sufficient food and space tend to grow exponentially. This is a generalization that is made for simple systems. The generalization implies that the birth rate is higher than the death rate. Birth rate is defined as the ratio of births to the number of organisms in a population over a given time period. Death rate is defined as the ratio of deaths to the number of organisms in a population over a given time

period.In more complex systems, additional factors influence the population size.Individuals can immigrate into the ecosystem from neighboring ecosystems.Alternatively, individuals in the population can emigrate out of the ecosystem. Immigration is defined as the movement of individuals into a population or ecosystem. Emigration is defined as the movement of individuals out of a population or ecosystem.One can imagine that in a simple system, such as a dish containing a population of Paramecium, the population members cannot emigrate out of the dish, and other Paramecium cannot immigrate into the dish on their own.Therefore, the only factors that influence the growth of the population are "birth" rate (Paramecium divide instead of giving birth) and death rate.However, one can imagine that the population of the state of Michigan might be influenced by all of the population growth factors--birth rate, death rate, immigration, and emigration. Michigan's population will only grow if the birth rate and the immigration rate combined are greater than the combined death rate and the emigration rate.

Other Factors that Affect PopulationsOther factors can affect organism populations. These factors include natural causes, such as tornadoes, hurricanes, and diseases, changes in climate, the introduction of non-native species, and human activity.Typically, these factors cause a decline in the population sizes of native organisms.

Limiting FactorsLimiting factors are physical, biological or chemical factors that can affect the growth or diversity of organisms within an ecosystem. If there is a chemical spill that kills plant and animal life in an area, that ecosystem is dramatically impaired. Sometimes ecosystems can recover from limiting factors, and other times they cannot.

Carbon-Oxygen CycleBiogeochemical cycles are predictable pathways followed by chemical elements or molecules as the elements or molecules travel through the living and nonliving parts of an ecosystem.

Biogeochemical CyclesMatter is neither created nor destroyed. Instead, it is converted from one form to another. As it changes forms, it often moves among the Earth's biosphere, atmosphere, lithosphere, and hydrosphere.Carbon and oxygen are two of the forms of matter that move in cyclic paths through the Earth's layers. These cyclic paths are called biogeochemical cycles.

Photosynthesis & Respiration in the Carbon-Oxygen CycleThe carbon and oxygen cycles are sometimes discussed separately.However, these cycles can also be addressed together since they are interdependent, or reliant upon each other for proper operation. This combined, two-part, interdependent cycle is known as the carbon-oxygen cycle.There are two primary natural processes that drive the carbon-oxygen cycle:photosynthesis and cellular respiration. Each of these processes must take place in order for the other half of the cycle to function properly because the products of one process are the reactants of the other.

The black arrows represent the flow of oxygen and carbon dioxide due to cellular respiration. The magenta arrows represent the flow of

gases due to photosynthesis.

Photosynthesis: 6CO2 + 6H2O   C6H12O6 + 6O2

The products of photosynthesis are glucose and oxygen. These materials are the reactants of cellular respiration.Cellular Respiration: C6H12O6 + 6O2   6CO2 + 6H2OThe products of cellular respiration are carbon dioxide and water. These materials are the reactants of photosynthesis.

Other Processes in the Carbon-Oxygen CycleThere are several processes that, in addition to photosynthesis and cellular respiration, either store carbon dioxide or release it into the environment.These processes are summarized in the table below.

Process Natural or Man-made CO2 Released or Stored

photosynthesis natural stored

cellular respiration natural released

erosion both released

combustion of fossil fuels man-made released

burning of forests both released

sedimentation & compaction natural stored

decomposition natural released

The Nitrogen CycleThe nitrogen cycle is the cycle of consumption and regeneration of nitrogen within our environment.

OverviewNitrogen is an essential component of amino acids (proteins) and nucleic acids (DNA and RNA). Therefore, all organisms require nitrogen to survive.Even though nitrogen is the most abundant gas in the atmosphere, most organisms are unable to use this form of nitrogen. However, there are a few microscopic organisms and natural processes, such as lightening, that can convert unusable nitrogen in the atmosphere to usable forms of nitrogen.

Image is courtesy of the EPA.

During the nitrogen cycle, atmospheric nitrogen (N2) is fixed or changed into nitrogen containing compounds, such as ammonia or nitrates, by nitrogen-fixing bacteria. Plants can then absorb the nitrogen compounds from the soil and use it to form chlorophyll and other important biological molecules.Consumers must obtain nitrogen from the organisms they consume.Herbivores receive their nitrogen from the plants that they eat, and carnivores get their nitrogen from the animals they consume. However, all organisms depend on the ability of nitrogen-fixing microbes to convert atmospheric nitrogen into a form of nitrogen that plants can assimilate, or take in and use.

Finally, nitrogen is returned to the atmosphere through the combustion of fossil fuels or when bacteria or fungi break down the nitrogen found in fertilizers, urine, and dead plants and animals.

Water CycleWater is constantly being recycled through the water cycle, or the hydrologic cycle. The water cycle describes the continuous movement of water on, above, and below the surface of the Earth. 

The amount of water on Earth remains constant, but it continuously changes forms as energy from the Sun drives the cycle.

The water cycle describes how water moves from one stage of the cycle to the next. Actually, there is much more water being stored in the cycle than is moving through the cycle. Water may be stored for a short time as water vapor in the atmosphere, for days or weeks in a lake, or for thousands of years in a polar ice cap.

The water cycle is a cycle with no beginning or end. It includes the following processes:Condensation is the changing of gas to a liquid (water vapor to water) and is crucial for the formation of clouds. Clouds form in the atmosphere when air containing water vapor rises and cools. Water vapor can be present in the air even when clouds are not visible. Clouds become visible when the water molecules combine with other water molecules and tiny particles and form cloud droplets.Water returns to the Earth as precipitation. Precipitation is the process by which water molecules condense to form drops heavy enough to fall to the Earth’s surface. During infiltration, also called percolation, water fills the porous spaces in the lithosphere. Surface runoff occurs when no more water can be absorbed into the ground, and gravity pulls it downhill. Water flows over land and forms rivers. Rivers usually flow into the ocean, and the water cycle continues from there.Evaporation and transpiration are similar in that they are both processes in which water is changed into water vapor. Evaporation often happens as a result of heat – liquid water is heated until it turns to a gas, water vapor, and is released into the atmosphere. Transpiration is the process by which water is carried through plants, from roots to leaves, where it changes to water vapor and is released to the atmosphere.Sublimation is the changing of water from a solid directly to a gas with no intermediate liquid stage. The opposite of sublimation is deposition, when water vapor changes directly to a solid – ice. Snowflakes and frost are examples of deposition.

Natural Environmental ImpactThe Earth has continually changed since its formation. In addition to human activities, there are many natural processes that are involved in shaping the environment.

VolcanoesVolcanic eruptions release many tons of greenhouse gases into the atmosphere. These can contribute to an increase in global temperature.

However, volcanoes can also release particles such as ash into the atmosphere. These particles reflect sunlight back into space, keeping it from reaching the Earth. Particles such as ash and dust can actually lower the temperature of the Earth.Depending on the amount of particulate matter ejected into the atmosphere during an eruption, these temperature drop can last from days to even months.

Image is courtesy of the USGS/Cascades Volcano Observatory.

FireFires are a natural part of the life cycle of ecosystems such as forests and grasslands. Though they are sometimes attributed to human activity, wildfires are also caused by natural phenomena. Lightning and volcanic eruptions are often causes of wildfires as well.The overall damage is dependent on factors. The build-up of underbrush can cause a fire to burn hotter and cause more damage. Drought conditions can also extend the amount of damage.

Image is courtesy of the US Fish and Wildlife Service.

Fires allow new plants to grow.Fires return the nutrients that were bound up in the plants to the soil, enriching it for new plant growth. Fire also clears out old growth that blocked the light, giving new plants a chance to gain a foothold.

Fires destroy vegetation.Fires can destroy the food sources of herbivores. By destroying ground cover such as grass, they increase the erosion of top soil.

Fires destroy animal habitats.Many animals become displaced while fleeing fires or are killed by the smoke, fire, or destruction of their food sources.

Fires release ash into the air, which can increase cloud formation and block sunlight from reaching the Earth.

PlantsPlant life has a tremendous effect on the environment.

Plants prevent erosion.Plants' roots anchor soil in place.They also absorb water, reducing the amount of runoff that can carry away soil.

Plants improve air quality.Photosynthesis removes carbon dioxide, a greenhouse gas, from the air, and releases the oxygen that animals need to live. Plants can also remove some pollutants from the air.

Plants improve soil quality.Many plants release chemicals from their roots that improve the quality of the soil. Some plants can even remove toxic metals from the soil.

Human Environmental ImpactHumans have the ability to affect the environment in many different ways. Construction, pollution, resource use, and preservation are just a few of the human actions that affect the environment.

Human Activity & Earth's ProcessesHuman activities can change the balance in Earth's processes, such as the carbon cycle and the water cycle. Careless human activity can also alter or destroy habitats and damage ecosystems.The Carbon Cycle & Climate Change

The use of fossil fuels has impacted the carbon cycle. When fossil fuels are burned, carbon is transferred from the geosphere to the atmosphere.Greenhouse gases, such as carbon dioxide, methane, and water vapor, are released into the atmosphere in large quantities when fossil fuels are combusted.The Earth's atmosphere naturally contains greenhouse gases, which trap sunlight energy. The increasing amount of greenhouse gases released by the growing population of humans is causing average global temperatures to rise, according to climate scientists.

The image on the left represents the natural greenhouse effect. The image on the right shows the effect of extra greenhouse gases on heat

retention within Earth's atmosphere.

Image courtesy of NPS.

Earth's Atmosphere

Human activities release substances into the air, some of which cause problems for humans, plants, and animals. Some air pollutants return to Earth in the form of acid rain and snow, which corrode statues and buildings, damage crops and forests, and make lakes and streams unsuitable for fish and other plant and animal life.One type of air pollution, known as particulate matter, consists of the small particles released into the air from burning fuel. These particles are very small pieces of matter, which are easily inhaled deeply into the lungs where they can be absorbed into the bloodstream or remain embedded in the lungs for long periods of time.

Burning fossil fuels also releases other pollutants, such as sulfur dioxide and mercury from burning coal and natural gas. Sulfur dioxide and nitrous oxide compounds that are released into the atmosphere contribute to acid rain.Some substances that are considered air pollution by the U.S. Environmental Protection Agency include: Ozone

Carbon Monoxide

Carbon Dioxide

Nitrogen Oxides

Sulfur Dioxide

Lead

Mercury

Particulate Matter

Ozone is a type of gas in the atmosphere that shields the Earth from the Sun's radiation. Near ground level ozone is considered a pollutant—it is a major component of smog—but in the upper atmosphere it works a bit like sunblock on a global scale.Chemicals called chlorofluorocarbons, or CFCs used in air conditioners and aerosol sprays destroy ozone. Scientists believe that these chemicals have caused two holes to develop in the ozone. These gaps in the ozone change both the amount and the type of solar energy that reaches Earth's surface.People worldwide are currently reducing the amount of ozone-destroying chemicals that are used.The Nitrogen Cycle

Human-made fertilizers are produced using nitrogen from the atmosphere.This nitrogen is converted into nitrates, just as would happen in the nitrogen cycle in nature. The creation and use of fertilizers both bypasses the natural nitrogen cycle, and cycles nitrogen more rapidly between the atmosphere and plants.The Water Cycle

Human activity alters the water cycle also. Surface runoff increases where more ground is covered with buildings and concrete. Pollution from these surfaces ends up in the water and changes the chemical balance of water bodies. Pollutants also leach from soils that have been polluted through spills or garbage, increasing the number of pollutants in waterways.

Landfills, which are humankind's most common way of disposing of solid waste, also impact the water cycle. Landfills often contaminate surface and

groundwater. 

Landfills are collection sites for solid waste materials. Water that is contaminated from the various organic and inorganic substances with which it comes into contact as it migrates through the waste is known as leachate. Water moving through a landfill inevitably becomes contaminated as leachate, which is undrinkable and often toxic.Most modern landfills in the U.S. have leachate collection and monitoring systems. These systems generally involve a layer of durable plastic that is placed beneath the waste to prevent water from seeping into the surrounding soil. But all such systems eventually fail, allowing nearby water sources to become contaminated over time.The Cycling of Energy: Food WebsThere is a natural energy flow between plants and animals, in which plants use sunlight energy to produce the food that animals use as an energy source.Human activity has impacted the cycling of energy too.When insecticides are used heavily, they can disrupt the food web in an ecosystem. When too many members near the bottom of a food pyramid, larger animals do not have enough food and their populations decline.The flow of energy is also disrupted by the removal of plants. Deforestation removes the link between solar energy and food for other organisms.Deforestation can greatly limit the energy entering into a food web.In other cases, human action increases the cycling of energy. Humans can build tools like greenhouses that allow plants to grow all year. Greenhouses increase the energy available to the plants, resulting in more plant growth and more food energy.

Human Activity & Earth's EcosystemsChanges caused by humans can upset the balance in an ecosystem. It is a simple fact that humans and animals need space to live. As more land is developed for human use, there is less land available for animals.Human Population Growth

Starting in the 1800s, humans have produced ever increasing amounts of chemical waste from technological activities. This began in connection with the Industrial Revolution, which started in the 1700s, and allowed for greater food production, helping to increase human health and lifespan.Medical advancements have also allowed humans to live longer lives.Because of this, the human population has increased dramatically in the last 200 years. The current human population is estimated to be about 6.6 billion.Animal Habitats

Human population growth has also resulted in changes to animal and plant population. Humans cultivate certain plants and animals for food, but pollution, deforestation, and land clearing have resulted in the loss of or changes to native animal habitats. This has caused some species to become threatened or endangered.Erosion

Erosion is a process that occurs naturally. However, the clearing of land through deforestation rapidly increases the amount of erosion. Without forest plants to trap and absorb water, erosion increases. Often very fertile, healthy soil is lost after deforestation.Acid Rain

Acid rain occurs naturally, but pollution from human activities can increase the acidity of rain significantly. Acid rain occurs when atmospheric pollutants become part of clouds and then fall as rain. The pollutants can come from volcanic eruptions, or they can come from human pollution of the atmosphere.Acid

rain can harm plants and alter water ecosystems. The death of plants due to acid rain can also increase erosion.The main way in which human activities generate acid rain is through the combustion of coal with high sulfur contents. When this coal is burned, SO2 and nitrous oxides are emitted as byproducts. These compounds then react with other substances in the atmosphere, and can then fall as acid rain.

This image is courtesy of the U.S. Environmental Protection Agency

Acid rain can cause lakes to become too acidic to support life, leach essential minerals out of the soil, and damage or kill trees. Although acid rain is very harmful to the environment, its chemical components do not generally become concentrated in animals.Some ecosystems are more resistant to acid rain than others. Lakes that are high in limestone and calcium carbonate can feel the effects of acid rain less than other environments. This is because limestone and calcium carbonate can neutralize the acid rain.

Human Activity & ConservationConservation is the careful use and preservation of Earth's natural resources.Conservation is based on the idea that people should plan ahead to minimize negative impacts. If people consider what impact their choices have on ecosystems first, they can look for ways to minimize that impact.Although there are no easy solutions to the many negative environmental impacts that human activities can have, there are, in fact, many human activities that can have a positive impact on the environment. Some of these activities can be summarized simply as reducing, reusing, and recycling.Reducing is the act of consuming fewer natural resources and decreasing the amount of waste a person creates. Reducing can come in many forms.Automobile manufacturers, for example, can reduce the amount of materials they consume by simply manufacturing smaller cars. Consumers can reduce the amount of fossil fuel energy they consume by operating smaller cars.Energy-efficient appliances and home heating systems also reduce the amount of resources needed to operate them.Reusing can be giving something a different purpose, such as using an old glass jar to store extra nails and screws. It can also be the act of passing something along once you are done with it. For instance, giving clothes you've outgrown to charity or to your younger brother or sister is reusing. It keeps the clothes out of

the landfill, and it also cuts down on the amount of new waste created in the production and purchase of new clothes.Recycling is the process of making new products from products that have been used before. Since fewer new resources are needed to make the recycled products, resources are conserved. For instance, recycling paper helps conserve trees. Recycling aluminum cans helps to conserve aluminum.

There are some materials that are easier to recycle than others. Things like glass bottles, plastic milk jugs, and newspaper are easy to recycle because each is made of mostly one kind of material.Other materials cannot be recycled by modern processes. As a result, these materials simply accumulate in the environment after their use. Light bulbs and some kinds of plastics are not recycled. Some of these items have recycling processes that are expensive. Others have processes that use a lot of energy or produce large amounts of waste.

Invasive SpeciesInvasive, or non-native, species may lead to extinction of native organisms by taking food, water, and space resources that the native organisms need in order to survive.

In the past few hundred years, humans have started to travel all over the world in boats, cars, and airplanes. Travelers may not know it, but they are often carrying plants, animals, and microorganisms with them when they travel.In many cases, non-native organisms do not survive well in the new environment. However, there are cases in which the invading species is able to survive and reproduce in the new habitat. And in some instances, they even do well enough to outcompete native species for food and other resources.This can lead to the decline or even destruction of native populations.One example of a successful invasive species is the kudzu plant. It is a vine with broad, heart-shaped leaves. Kudzu was brought to the U.S. from Japan.In the 1930s, the U.S. government began to promote its use to prevent soil erosion in areas without much vegetation.

Kudzu vine has covered these trees and bushes so thickly that it is difficult to see the trees underneath the vine growth.

Image is courtesy of Wikipedia

The kudzu plant has been incredibly successful in the climate of the southeastern United States. In many places there, kudzu has grown so well that it has crowded out native species. The kudzu completely covers native trees, grasses, and shrubs. Eventually, the native plants and trees die because they do not receive enough sunlight.